An SCR system incorporating an SCR device that has been developed is an exhaust gas purification system that purifies exhaust gas of a diesel engine of NOX.
The SCR system is to purify exhaust gas of NOX by supplying urea solution stored in a urea tank to an upstream part of the exhaust gas and reducing the NOX on an SCR catalyst with ammonia derived from the urea solution by the heat of the exhaust gas (see Patent Literature 1, for example).
A process of deriving ammonia from urea solution will be described below with reference to FIG. 4.
As shown in FIG. 4, as the temperature of the urea solution rises beyond the boiling point, water starts evaporating, and the urea solution transforms into melt and then to gas. The resulting gas crystallizes into solid as the temperature further rises. The resulting solid then sublimates into ammonia. Part of the melt and gas are hydrolyzed with stream to form ammonia and carbonic acid gas.
The urea solution is injected through a dosing valve (a urea injection device or a dosing module) provided to be exposed in the exhaust pipe at the upstream side of the SCR device.
As shown in FIG. 5, a dosing valve 104 includes a cylinder 127 having an injection orifice 128 and filled with urea solution at high pressure, a tubular plunger 130 having a slit 133, and a valving element 129 to close the injection orifice 128 that is attached to the plunger 130. A coil 131 is energized to pull up the plunger 130 to separate the valving element 129 from the injection orifice 128, thereby injecting the urea solution. Furthermore, a spring 132 is provided in the dosing valve 104 to bias the valving element 129 to be normally closed via the plunger 130. Therefore, when energization of the coil 131 is stopped, the plunger 130 is pushed down by the spring to make the valving element 129 close the injection orifice 128, thereby stopping injection of the urea solution.
The dosing valve 104 is provided in the exhaust pipe, through which exhaust gas at high temperature flows. Therefore, as the temperature of the dosing valve 104 rises, the temperature of the urea solution in the dosing valve 104 also rises. As a result, the urea solution in the dosing valve 104 crystallizes and adheres to the valving element 129 as shown in FIG. 6(a), or the urea solution crystallizes into a solid 134, and the solid 134 gets caught between the valving element 129 and a valve seat on the cylinder 127 to make it impossible to stop injection of the urea solution as shown in FIG. 6(b), thereby leading to malfunction of the dosing valve 104 and failure of proper control of injection of the urea solution.